What is in your current desktop PC, in terms of the CPU, memory, disks and GPU? If those components are fairly recent and higher-end, then you may be able to hold onto your current config for a while longer. That's because in terms of the total cost of this platform upgrade, that i7-7800K may not be sufficiently more powerful than your current CPU platform to justify the total cost of this upgrade - CPU, motherboard and RAM combined. In fact, you might have to spend even more than what the i7-7820K costs (CPU-wise) to get a sufficiently more powerful CPU that justifies such a high total cost.

And if your current PC uses DDR3 RAM, you will have to replace that in addition to the CPU and motherboard since the X299 boards require DDR4 RAM, and you cannot carry over your existing DDR3 RAM to such a new build.